Advancing soil health with biochar - Effects on soil microbial activity and diversity

Improving soil quality is essential for sustainable agriculture and biochar is a promising soil amendment because its effects on soil chemistry and microbial functioning depend strongly on feedstock and production conditions. This study compared wheat straw biochar (WSBC) and pine wood biochar (PWBC) by characterising their physicochemical and safety properties and evaluating their effects on agricultural soil during a six-month incubation at 1, 2.5 and 5% (v/v) amendment rates. Soil physicochemical properties, total heterotrophic bacteria, dehydrogenase activity and community-level physiological profiles were determined. Both biochar samples met international safety standards for the parameters tested. WSBC had higher ash and nutrient contents, higher pH and higher electrical conductivity than PWBC and exerted the stronger short-term effect on soil properties. WSBC increased soil pH from 6.6 to 7.1 at the 2.5% application rate, raised electrical conductivity from 300 to 570 μS/cm at 5% and increased plant-available K from 289 to 849 mg/L. PWBC caused only minor changes in soil pH, electrical conductivity and K. Total Kjeldahl N, extractable P, total organic carbon and organic matter remained relatively stable across treatments. Total heterotrophic bacteria declined by about three log units by day 30 in all soils, indicating no specific biochar effect on bacterial abundance. Dehydrogenase activity peaked on day 90, increasing 4.3-fold at 5% WSBC and 1.95-fold at 5% PWBC relative to the control. By day 180, microbial metabolic profiles showed increased utilisation of more chemically complex substrates, particularly polymers and surfactants. These results indicate that WSBC acts primarily as a liming and nutrient-supplying amendment that stimulates microbial activity, whereas PWBC behaves as a more inert, carbon-rich material with limited short-term effects on soil chemistry. Biochar addition did not disrupt soil biological functioning, but promoted time-dependent shifts in microbial metabolism.